Prius tank size?

I wonder if there's a buffer programmed in there that lessens over time. I got mine at 50K miles and thought it was a 9 gallon tank - it never took more than 8.5 gallons after it said "Add Fuel". Then it started taking over 9 gallons, then 10 gallons, and yesterday 10.5 gallons. It's over 100K now. I thought gas stations were trying to rip me off by speeding up their counter. I filled a 2.5 gallon tank also and it came to the correct mark on the tank. I suspect it's just been telling me there's less in the tank than there really is. My theory is that when it is full, it's really full, so don't put more in. But, when it's "empty" it really not empty, although who wants to test that theory?

 

The Gen II Prius has a rubber inflatable bladder in the gas tank. The bladder makes an attempt to fill the empty space in the tank. This reduces the amount of fuel vapors that can escape into the air. The amount of fuel you can add is dependant on how well the bladder retracts. It's just one of those things you have to live with when you own a Prius.

 

Seeing as the fuel fills the bladder, it might be more accurate to say that the amount of fuel you can add is dependent on how well the bladder expands.

So you have 2 issues working against you when trying to figure out what is going on. First of all, depending on how much the bladder expands, the tank won't always hold the same amount of fuel. After one fill-up you might have 11.5 gal, whereas after another fill-up you might only have 10.8. Secondly, many have found that the fuel gauge doesn't give a good indication of the exact amount of fuel left in the tank. When it is down to it's last flashing pip, you might have 0.5 gallons, or you might have 1.7 gallons. Either way you know that fuel is low and it wouldn't be a bad idea to fill up. If you typically fill at 2 or 3 pips, it wouldn't surprise me that you are getting 6 to 8 gallons to fill the tank.

 

Because the flashing pip does not register a definite amount of fuel left in the tank I would not suggest driving more than a few miles once it starts flashing. I've read some reports of drivers going over 50 miles once the flashing starts. I've also read about drivers going less thank 10 miles then running out of gas.

It is generally suggested that you fill up when you are down to 2 pips. Sometimes I get as low as one.

 

I filled my wife's '05 yesterday at two pips. It took 10.1 gals. I almost never get more than 9.5 gals in and that's usually at one pip left. The OAT yesterday at fill-up was 97F. I'm thinking the warm temps allowed the bladder to expand much more than usual.

 

Wow! I've never put more than 9.6ga in mine. That was after driving for 537 miles and about 17 miles on the flashing low fuel light. Mpg for that tank was 55mpg on the MFD and by hand calculation.

 

FWIW, I've never put more than 9.975 gallons in mine.

 

Safe is a very subjective term. How far will the nearest gas station be when it starts flashing? How likely are you to immediately notice when it starts flashing? How likely are you to end up in a situation where you don't have time to fuel up when it starts flashing? Do you (or will you) carry extra fuel with you in case there is less in the tank than you thought once it starts flashing?

Keep in mind what I said:

You'll know you are low on fuel, but you won't really know exactly how low.

In general, I use the following plan:

5 pips or less = get fuel if it seems cheaper than usual, if I'm planning a long drive in the near future, or if I expect to drive where I don't know where the gas stations are.

2 pips = Get fuel if it is convenient

1 pip = Get fuel even if it is inconvenient

flashing pip = GET FUEL NOW at the nearest gas station!